1. Field of the Invention
The present invention relates to a transmitter and a receiver for spread-spectrum communication employing a direct spreading method, and also relates to a spread-spectrum communication system.
2. Description of the Related Art
In a spread spectrum communication system employing a direct spreading method in which digital information to be transmitted is directly spread with a digital code sequence, when one period of the code sequence is allotted to one bit of information, the bandwidth used expands as the information rate to be transmitted increases, as long as the bit length in one period of the code sequence is fixed. On the other hand, if the information rate is to be increased within a predetermined bandwidth, the expansion ratio must be lowered by reducing the bit length of the code sequence. In other words, in spread-spectrum communication employing the conventional direct spreading method, with a given bandwidth and expansion ratio (bit length of spreading code) the information rate cannot be increased above a certain value.
Japanese Unexamined Patent Publication No. 7-46222 discloses a communication system wherein the code phase of a code sequence for directly spreading the information to be transmitted is modulated in accordance with another information or the code sequence itself is phase-modulated for transmission, thereby achieving the transmission of larger volumes of information than is possible in spread-spectrum communication employing the conventional direct spreading method under conditions of a predetermined bandwidth and spreading code length. In this system, since the code phase in the received signal varies due to the modulation, synchronization cannot be maintained by establishing code synchronization and chip synchronization by computing the correlation between the received signal and the despreading code at the receiving end, as is done in the conventional spread-spectrum communication. Tn the above patent publication, therefore, synchronization is established by transmitting a special bit pattern in both directions before commencing a communication, and after the synchronization is established, the synchronization is maintained by synchronizing the code sequence at the receiver with the code sequence at the transmitter in the opposite direction.
The above prior art has a problem in that a bit pattern for establishing synchronization needs to be transmitted every time a communication is commenced. Another problem is an inability to cope with phase shifts caused by minute frequency variations after the establishment of synchronization.